1. Technical Field
The present invention relates to fuel gas reforming method and apparatus for a power generation system using a fuel cell which directly converts chemical energy of fuel into electric energy, and particularly to reforming method and apparatus which can improve a reforming conversion ratio of the fuel to raise a power generation efficiency.
2. Background Art
A power generation system using a molten carbonate fuel cell stack is known in the art. Generally, the molten carbonate fuel cell stack includes a plurality of cell elements and each cell element includes an electrolyte plate (tile), a fuel electrode and a oxygen electrode (air electrode). The electrolyte plate of the molten carbonate fuel cell is made from a porous material soaked with a molten carbonate. The fuel and oxygen electrodes sandwich the electrolyte plate, and a fuel gas is supplied to the fuel electrode and an oxidizing gas is supplied to the oxygen electrode for power generation. A plurality of cell elements are piled up with separators being interposed between each two adjacent cell elements.
Conventionally, a hydrocarbon such as methane, LPG and naphtha is used as a raw material gas for the fuel gas fed to the fuel electrode of the molten carbonate fuel cell power generation system. The hydrocarbon is reformed with steam to obtain the fuel gas. In the reforming operation, the steam is mixed with the raw material gas in the mol ratio of 2-4:1 (steam:raw material gas) and the mixture is heated to a temperature of 600.degree.-900.degree. C. with a reforming catalyst. The mixture contacts the reforming catalyst during the reformation.
On the other hand, preheated air or exhaust gas (the fuel gas is burned and used as a heating source for the reformer and a combustion gas is discharged from the reformer as the exhaust gas) is fed to the air electrode as the oxidizing gas.
A fundamental operation of a conventional molten carbonate fuel cell is shown in FIG. 9 of the accompanying drawings. A raw material gas C.sub.4 (C.sub.4 is used as a representative of the raw material gas in the following description.) is reformed to H.sub.2 and CO by a reformer 1 and the fuel gas FG (H.sub.2 and CO) are supplied to a fuel electrode 4 from the reformer 1. Meantime, an oxidizing gas OG is fed to an oxygen electrode 3 and a following reaction takes place: EQU CO.sub.2 +1/2O.sub.2 +2e.sup.- .fwdarw.CO.sub.3.sup.-
Upon this reaction, a carbonate ion CO.sub.3.sup.- is produced and the carbonate ion CO.sub.3.sup.- migrates in the electrolyte plate 2 to reach the fuel electrode 4. Approximately at the same time, the fuel gas FG is fed to the fuel electrode 4 to cause a following reaction: EQU CO.sub.3.sup.- +H.sub.2 .fwdarw.H.sub.2 O+CO.sub.2 +2e.sup.-
The gas from the reformer 1 only contributes to the reactions in the molten carbonate fuel cell I and CH.sub.4 (raw material gas) is not reactive. Therefore, the ratio of CH.sub.4 in the fuel gas FG supplied to the fuel electrode 4 directly influence the power generation efficiency of the fuel cell. More specifically, it is desired to reduce the amount of methane remaining after the reforming reaction (remaining raw material gas) as small as possible. In other words, it is desired to raise the reforming conversion ratio to 100% as close as possible.
During the methane reforming reaction in the reformer 1, various reactions occur simultaneously, but actually considering following two major reactions are satisfactory: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 ( 1) EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 ( 2)
It is known that the amount of CH.sub.4 existing after the CH.sub.4 reforming reaction is reduced as a reaction pressure drops and/or a reaction temperature rises. The reforming reaction (1) is an endothermic reaction and the CO shift reaction (2) is an exothermic reaction, but the reaction in the reformer 1 is an endothermic reaction as a whole. Therefore, it is required to heat the reformer.
In terms of thermal economy, the ratio of steam to CH.sub.4 (S/C ratio: "R" is used to represent this ratio in the following.) should be maintained as low as possible to obtain CH.sub.4 of desired concentration. In a case where the CH.sub.4 concentration is 10% at the exit of the reformer, a relation between the pressure and the temperature draws a curve as shown in FIG. 11 of the accompanying drawings, with a parameter being R.
It is understood from FIG. 11 that the reforming temperature should be raised to realize a constant CH.sub.4 concentration if the R is fixed and the reforming pressure is raised. Therefore, generally a high nickel alloy (e.g., 25Cr--20Ni) is used as a material for the reformer 1 to bear a high temperature and a high pressure. However, the elevated temperature considerably reduces the longevity of the material. Generally, 950.degree.-1,000.degree. C. is considered as a maximum temperature for the material.
Since the reactions (1) and (2) in the reformer proceed almost simultaneously, they can be combined to a following equation (3) for simplification and such an approximation does not affect the principle of the invention: EQU CH.sub.4 +2H.sub.2 O.fwdarw.CO.sub.2 +4H.sub.2 ( 3)
In the following description, it is supposed that the reaction (3) takes place in the reformer 1. The increase of CO.sub.2 upon the reaction in the fuel cell I rather promotes the reforming reaction, and calculation results show that the reforming becomes easier if the increase of the CO.sub.2 is taken in account than not. Therefore, the increase of CO.sub.2 is neglected in the following description since such neglectedness does not affect the effectivity of the invention.
The power generation efficiency A of the fuel cell power generation system is determined by a product of the reforming conversion ratio B of the reformer 1 and a power generation factor C of the fuel cell I of FIG. 9. This can be expressed by a following equation: EQU A=B.times.C
Therefore, it is desired to raise the reforming conversion ratio B as high as possible to raise the power generation efficiency of the fuel cell power generation system (total power generation efficiency) A. To raise the reforming conversion ratio B, however, the pressure or the temperature should be raised, as mentioned earlier.
On the other hand, raising the system pressure results in raising the power generation factor C of the fuel cell I, as shown in FIG. 12 of the accompanying drawings. However, the reforming conversion ratio B drops as the system pressure rises.
In a case where the reforming temperature is maintained at 780 .degree. C., for example, the reforming conversion ratio B drops with the rising pressure, as shown in FIG. 13 of the accompanying drawings. As a result, the total power generation efficiency A is deteriorated. At the pressure of 8 ata, the reforming temperature should be 865.degree. C. or more and the value (reforming conversion ratio) of the reforming conversion ratio B should be 0.96 (96%) or more to obtain the total power generation efficiency A better than a case of 3 ata. To realize the 99% reforming conversion ratio, the reforming temperature should be further raised. This raises problems relating to the material and structure of the reformer 1.